In the field of four-stroke self-ignition engines, variable valve actuating mechanisms that can change the valve timings of the intake valves and the exhaust valves with a good response have been proposed. See JP2012-7596A, for instance. The variable valve actuating mechanism proposed in this patent publication is applied to an OHV (over head valve) engine in which upper rocker arms provided above the cylinder head for actuating the intake valve and the exhaust valve are actuated by a camshaft provided in the crankcase via pushrods. The variable valve actuating mechanism is provided in association with lower rocker arms interposed between the camshaft and the pushrods.
More specifically, the variable valve actuating mechanism disclosed in JP2012-7596A comprises a composite camshaft consisting of a first camshaft rotatively actuated by the crankshaft via a timed transmission mechanism and provided with low profile cams and a second camshaft passed coaxially through the first camshaft and provided with high profile cams. The second camshaft is rotatably supported by a fixed part of the engine, and the first cam shaft is rotatably supported by the second camshaft. An axial end of the first camshaft is provided with an engagement recess while the adjoining part of the second camshaft is provided with a clutch member that can be selectively placed in an engaged position for connecting the first and second camshafts at a fixed phase relationship and a disengaged position for disconnecting the second camshaft from the first camshaft. Thus, the high profile cams determine the cam lifts when the two camshafts are connected to each other, and the high profile cams on the second camshaft become ineffective when the second camshaft is disconnected from the first camshaft, leaving the low profile cams to determine the cam lifts.
This previously proposed variable valve actuating mechanism however suffers from the problem of high complexity as the two separate camshafts each provided with cams are required, and have to be combined coaxially in a highly complex manner. Therefore, this mechanism is not suitable for small engines such as uni-flow two-stroke engines, and a more simple and compact variable valve actuating mechanism is desired.